<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <iframe
      src="http://b.zf1.cn:3000/b.html"
      frameborder="0"
      onload="load()"
      id="frame"
    ></iframe>
    <script>
      document.domain = "zf1.cn";
      function load() {
        console.log(frame.contentWindow.a);
      }
    </script>
  </body>
</html>
